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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 29 Sep 2021 16:28:40 -0700
Message-ID: <CAHk-=wh0BNEDz+uOdJWG8iW=n0PeOEjZpHyuSN2g0pKSCj+6iQ@mail.gmail.com>
Subject: Re: [PATCH] modpost: add allow list for llvm IPSCCP
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Arnd Bergmann <arnd@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev
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On Wed, Sep 29, 2021 at 3:59 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> +static const struct secref_exception secref_allowlist[] = {
> +       { .fromsym = "__first_node", .tosym = "numa_nodes_parsed" },
> +       { .fromsym = "__next_node", .tosym = "numa_nodes_parsed" },
> +       { .fromsym = "__nodes_weight", .tosym = "numa_nodes_parsed" },
> +       { .fromsym = "early_get_smp_config", .tosym = "x86_init" },
> +       { .fromsym = "test_bit", .tosym = "numa_nodes_parsed" },
> +};

This list is basically made-up and random.

Why did those functions not get inlined? Wouldn't it be better to make
them always-inline?

Or, like in at least the early_get_smp_config() case, just make it be
marked __init, so that if it doesn't get inlined it gets the right
section?

It seems silly to add random source mappings to a checking program.

It was bad for the gcc constprop hack, but at least there it was a
clear case of "this inlining failed". This ad-hoc list has cases of
things that are clearly wrong in general ("test_bit()" must not use
initdata), and that "ok, the function just doesn't have the right
section marker.

(All of get_smp_config/early_get_smp_config/find_smp_config should be
__init, since they most definitely cannot work after __init time - but
why a compiler doesn't just inline them when they are one single
indirect call, I don't really get)

         Linus